When glass melt is cooled rapidly from high temperature, the movement of atoms within the cooling liquid slows down with decreasing temperature and eventually diminishes to oscillations about fixed positions due to normal thermal population of vibrational states. These positions are typically not those that would be adopted were the glass to be held for an extended period of time (ranging from seconds to days) at intermediate temperatures (e.g., the glass transition temperature or the strain or anneal points). As a consequence, when a rapidly quenched glass is reheated to intermediate temperatures, the thermally-populated vibrational states allow for relaxation of atoms into positions that better satisfy their individual and collective bonding requirements. Since this is typically accompanied by an irreversible decrease in the physical dimensions of a bulk piece of glass, thermal relaxation upon reheating is said to produce compaction of the glass.
A typical low-temperature polysilicon (LTPS) process includes steps that raise the glass substrate temperature to 600° C. or more. This is close to the strain point of typical active matrix LCD (AMLCD) display substrates, and thus without an intermediate annealing step these glasses will substantially compact in the low-temperature polysilicon process. This is highly undesirable, as it severely impacts the registry of pixels between the TFT and color filter sheets, and also can impact the performance of the TFTs themselves.
It has been found that certain glass sheets made by fusion down-draw processes that are suitable for TFT displays based on amorphous silicon technology exhibited too large a compaction to be used for those based on LTPS technology. Therefore, there exists a genuine need for a glass sheet with a pristine surface suitable as glass substrates for LTPS TFT display fabrication, and an economical process for making such glass sheets.
The present invention satisfies this need.